Before humanity changed the climate, shifting climates changed humanity, a new study has found. Although the conclusion is in line with expectations, it took a truly immense amount of computing power to provide the evidence. The work has already provided a basis for answering previously much more uncertain questions about our evolution.
We can see from the species around us that animals respond to changing climatic conditions by adjusting their range. Anthropologists have long suspected the same was true for human ancestors. However, climate records from the sites where ancient human fossils have been found are sparse, and the sites themselves potentially unrepresentative, leaving the climatic influence on our evolution speculative.
However, a team led by Professor Axel Timmermann of Pusan National University noted we don't need local proxies to know the climate at a particular site and time if we know what global conditions were like. Fine-scaled climate modeling has been restricted to much more recent timescales, but in Nature, Timmermann and co-authors have changed that. They ran Aleph, one of South Korea's most powerful supercomputers, for six months to create the first really detailed climate model that stretches back two million years. They then focused on conditions at locations where human fossils and tools have been found at the times they were found there, looking at rainfall and plant productivity, as well as seasonal temperatures.
“Even though different groups of archaic humans preferred different climatic environments, their habitats all responded to climate shifts caused by astronomical changes in earth’s axis wobble, tilt, and orbital eccentricity with timescales ranging from 21 to 400 thousand years,” Timmerman said in a statement.
The work required Aleph to produce 500 Terabytes of data, which the team notes would fill hundreds of hard disks.
To confirm their conclusions, Timmerman and co-authors shuffled the ages of the evidence of human presence. If climate was not affecting where humans chose to live, they reasoned, this wouldn't matter. Instead, they found Homo sapiens, Neanderthals and H. heidelbergensis disappeared from parts of their range when conditions became unsuitable and reappeared when the times suited them.
Having achieved the expected result, the authors then used their model to tackle more open questions, the first being when different human species habitats' might have overlapped. This can then be used to explore when there might have been contact, and therefore interbreeding, between species.
The authors concluded Homo heidelbergensis split into two lines, one in Eurasia and the other in Africa. Between 400,000-500,000 years ago the Eurasian line could have shared space with Neanderthals, and probably Denisovans, suggesting this is where those species evolved. Homo Sapiens emerged out of the southern African side of the human family 300,000 years ago, distinguished in part by an affinity for drier climates.
“Our climate-based reconstruction of hominin lineages is quite similar to recent estimates obtained from either genetic data or the analysis of morphological differences in human fossils, which increases our confidence in the results,” said co-author Dr Jiaoyang Ruan.
Looking further back, the team detected an important shift in our ancestors' preferences. “Early African hominins around 2-1 million years ago preferred stable climatic conditions. This constrained them to relatively narrow habitable corridors,” said Pusan National University PhD student Elke Zeller.
This changed around 800,000 years ago when H. heidelbergensis broadened their dietary preferences compared to their predecessors H. habilis and H. ergaster, and began expanding to a wider range of habitats. Subsequent generations of humans have maintained this adaptability, although a shortage of fossils led some, such as H. floresiensis to be excluded from the analysis.
Not only has this enabled humanity to occupy a vast diversity of locations, it has also benefited us as the pace of global climate cycles sped up. “We are who we are because we have managed to adapt over millennia to slow shifts in the past climate,” Timmerman said. Whether we can cope with changes we are inducing to the climate that operate at least 10 times faster than any of our ancestors faced remains to be seen.